Hmm. Googling images gets a lot of variety for common orache and fat hen, it seems on balance more like common orache with the wide start to the leaves but it does seem a bit more saw-edged than most of the pics, and it's pretty tall- pics don't make it that obvious I think. A good match though. Cheers!

Well, I'm going to let it grow a bit and see what it does, guess I will stop it from seeding though. I might have to do something more drastic about the Cameron infestation.